How to Fix Preorder Binary Tree Traversal Errors

In this tutorial, you'll learn about How to Fix Preorder Binary Tree Traversal Errors. We cover key concepts, practical examples, and best practices.

Fix preorder binary tree traversal errors when root processing order incorrect or right children lost.

Quick Fix

Wrong

def preorder(r):
    if not r: return []
    return [r.val]+preorder(r.left)+preorder(r.right)

Stack overflow on deep trees.

Right

def preorder(r):
    res,st=[],[r]
    while st:
        cur=st.pop()
        if cur:
            res.append(cur.val)
            st.append(cur.right); st.append(cur.left)
    return res

Input: [1,null,2,3] -> [1,2,3]. Iterative with stack.

Prevention

Push right before left so left processes first.

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